Flow, Turbulence and Combustion 72: 29–47, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 29 Leading-Edge Reaction Zones in Lifted-Jet Gas and Spray Flames S.K. MARLEY 1 , K.M. LYONS 1,⋆ and K.A. WATSON 2 1 Department of Mechanical and Aerospace Engineering, North Carolina State University, Box 7910, Raleigh, NC 27695-7910, U.S.A. 2 Department of Mechanical Engineering, University of the Pacific, Stockton, CA 95211, U.S.A. Received 4 April 2003; accepted in revised form 1 October 2003 Abstract. An investigation of the leading edge characteristics in lifted turbulent methane-air (gaseous) and ethanol-air (spray) diffusion flames is presented. Both combustion systems consist of a central nonpremixed fuel jet surrounded by low-speed air co-flow. Non-intrusive laser-based diag- nostic techniques have been applied to each system to provide information regarding the behavior of the combustion structures and turbulent flow field in the regions of flame stabilization. Simultaneous sequential CH-PLIF/particle image velocimetry and CH-PLIF/Rayleigh scattering measurements are presented for the lifted gaseous flame. The CH-PLIF data for the lifted gas flame reveals the role that “leading-edge” combustion plays as the stabilization mechanism in gaseous diffusion flames. This phenomenon, characterized by a fuel-lean premixed flame branch protruding radially outward at the flame base, permits partially premixed flame propagation against the incoming flow field. In contrast, the leading edge of the ethanol spray flame, examined using single-shot OH-PLIF imaging and smoke-based flow visualization, does not exhibit the same variety of leading-edge combustion structure, but instead develops a dual reaction zone structure as the liftoff height increases. This dual structure is a result of the partial evaporation (hence partial premixing) of the polydisperse spray and the enhanced rate of air entrainment with increased liftoff height (due to co-flow). The flame stabilizes in a region of the spray, near the edge, occupied by small fuel droplets and characterized by intense mixing due to the presence of turbulent structures. Key words: diffusion flame, double flame, entrainment, leading-edge, spray combustion. 1. Introduction Many lifted flame stabilization studies have shifted their focus to approaches that lie between the fully premixed argument introduced by Vanquickenborne and Van Tiggelen [35] and the diffusion flame quenching arguments proposed by Peters and Williams [28]. In particular, partially premixed flame propagation and, more specifically, triple flame arguments have received considerable attention and Peters [29] offers a thorough review. Triple flames consist of fuel-rich and fuel-lean pre- mixed branches in addition to an ordinary diffusion flame that extends downstream from the intersection of the two premixed zones (Figure 1). In theory, the premixed wings allow for flame propagation against the incoming unburned flow while sta- ⋆ Author for correspondence, E-mail: lyons@eos.ncsu.edu.